1. Field of the Invention
The present invention relates to a semiconductor integrated optical device, a manufacturing method thereof, and an optical module.
2. Description of the Related Art
In recent years, in order to increase a device function and downsize an optical module, there has been developed a semiconductor optical integrated device in which a laser portion configured to generate light and various functional elements are monolithically integrated on the same substrate (for example, JP 2008-277445 A).
As a method of manufacturing the integrated structure, there is known a method involving forming a mask in a partial region of a semiconductor laminated structure first formed by crystal growth, removing an unnecessary part by etching, and forming a different semiconductor laminated structure in the removed region by crystal growth again (hereinafter simply referred to as “regrowth”). The region in which different semiconductor laminated structures are connected to each other is formed such that the optical axes thereof are aligned with each other, and hence the above-mentioned method is generally called a butt joint (BJ). With use of the BJ, an optimal semiconductor laminated structure can be independently designed for each of the semiconductor optical elements to be integrated, thereby being capable of manufacturing a sophisticated semiconductor optical integrated device.
For example, as the semiconductor optical integrated device manufactured by the BJ, in JP 2009-004488 A, there is disclosed a BJ integrated device formed by BJ integrating a semiconductor laser and an electroabsorption modulator on the same substrate and forming a protective layer over the BJ connection, to thereby secure high reliability and yield.
Further, in JP 2010-045066 A, there is disclosed a distributed reflector (DR) laser as a BJ integrated structure for improving both of a single longitudinal mode oscillation yield and a high-speed characteristic of a distributed feedback (DFB) laser.
The DR laser is a semiconductor laser formed by BJ integrating a distributed Bragg reflector (DBR) mirror at a rear facet of the DFB laser. The DBR mirror includes a passive waveguide and a diffraction grating having the same period as the diffraction grating integrated in the DFB laser. Further, a non-reflective film is formed on each of a facet of the DBR mirror that is not connected to the DFB laser and a front facet (laser light emitting end) of the DFB laser. Further, the diffraction gratings have the same phase across the DFB laser region and the DBR mirror region.
As is well known, the single longitudinal mode oscillation characteristic of the DFB laser is significantly affected by the facet phase of the diffraction grating. However, the diffraction grating of the communication-waveband DFB laser is about 200 nm, and hence it is nearly impossible to precisely control the phase of the diffraction grating by cleavage, and thus randomness is caused. In contrast, the DR laser can remove the randomness of the diffraction grating phase that has been caused on the facet of the DFB laser by cleavage, and hence the single longitudinal mode oscillation yield can be increased.
Further, in a general semiconductor laser, reduction in volume of a region of an active layer configured to generate light enables increase in device-specific relaxation oscillation frequency, thereby being capable of improving the high-speed characteristic. As means for reducing the active layer volume simply without affecting the transverse mode and the performance of the active layer, shortening of an oscillator of the device is effective. However, the oscillator length is the device length in the DFB laser, and hence shortening of the oscillator causes difficulty in handling at the time of cleavage. In particular, in order to realize high-speed modulation of 25 Gbps or more, an oscillator length of a directly-modulated laser is required to be shortened to the vicinity of 100 μm, which makes this problem remarkable. In contrast, in the DR laser, the DBR mirror is BJ integrated, and hence a short DFB length required for increasing the speed and a handleable device length can both be attained.
In order to obtain stable laser oscillation in the DR laser, the diffraction gratings are required to be connected in the same phase without disconnection from the DFB laser region to the DBR mirror region.
Further, in JP 2014-082411 A, there is disclosed a DR laser having a structure in which the diffraction gratings are formed both above the active layer of the DFB laser region and above the passive waveguide layer of the DBR mirror region. In the following, such a DR laser is referred to as “upper diffraction grating DR laser” for distinction with the DR laser disclosed in JP 2010-45066 A.
In the upper diffraction grating DR laser, the active layer and the waveguide layer are epitaxially grown on a normal semiconductor substrate, and then the diffraction gratings are formed above those semiconductor layers. Therefore, a characteristic deterioration due to the surface morphology of the semiconductor substrate does not occur. Therefore, it can be said that the upper diffraction grating DR laser has a structure effective for a case where, for example, the diffraction grating is desired to be thickened to realize a high reflection coefficient.